The invention relates to a circuit arrangement for operating a discharge lamp, provided with
input terminals for coupling to the poles of a supply voltage source which delivers a low-frequency supply voltage, PA1 rectifying means provided with inputs coupled to the input terminals for rectifying the low-frequency supply voltage, and provided with output terminals, PA1 a branch D which interconnects the output terminals and which comprises a series arrangement of first unidirectional means D1, second unidirectional means D2, and third unidirectional means D3, the first and the second unidirectional means being shunted by first capacitive means C1, and the second and the third unidirectional means being shunted by second capacitive means C2, PA1 an ohmic resistor R which passes a current during lamp operation with which the first and the second capacitive means are charged, PA1 means M for generating a lamp current from the voltages present across the first and the second capacitive means.
Such a circuit arrangement is known from U.S. Pat. No. 5,387,847. The branch D and the first and the second capacitive means form an alternative to the use of a single buffer capacitance between the output terminals. Since the first and the second capacitive means are charged in series and discharged in parallel, the circuit arrangement has a higher power factor than if it contained a single buffer capacitance between the output terminals. The power factor of the circuit arrangement is further improved by the presence of the ohmic resistor R which limits the amplitude of the current with which the first and the second capacitive means are charged. A power factor higher than 0.95 can be obtained through a suitable choice of the components of the circuit arrangement. The circuit arrangement can be used in a wide range thanks to this high power factor value. For example, if the discharge lamp is a low-pressure mercury discharge lamp, the lamp current is often a high-frequency alternating current, and the means M often comprise one or several switching elements which are rendered conducting and non-conducting which high frequency during lamp operation. A certain amount of radio frequency interference (RFI) is generated thereby, flowing also into the supply mains while the rectifying means are in the conducting state. The first and the second capacitive means act as a filter via the second unidirectional means D2 whereby the amount of RFI is reduced. In the known circuit arrangement, however, the ohmic resistor R is placed in the branch D between the first and the third unidirectional means. The ohmic resistor does limit the amplitude of the current with which the first and the second capacitive means are charged in this position, but the filtering action of these capacitive means is at the same time strongly suppressed, so that the use of the known circuit arrangement generates a comparatively large amount of RFI in the supply mains.